“Although relations between Saudi Arabia, which is mostly Sunni, and Iran, which is predominantly Shiite, have never been warm, the tensions mark the worst deterioration in ties in recent years. It’s been some time coming: The two countries are on opposite sides of the civil war in Yemen, where the Saudis support the government and the Iranians the Houthi rebels; and in Syria, where Riyadh supports some rebel groups and Iran the government of President Bashar al-Assad. They have also parried in Bahrain, where a Sunni ruler governs a mostly Shiite population, and in Iraq, where Iran holds considerable influence over the predominantly Shiite government; and traded barbs over the death toll of the stampede at the Hajj last September.”